What is running economy?

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As scientific expressions go, ‘running economy’ (RE) has never been as familiar to runners as the terms 'VO2 max' and 'lactate threshold'. But now, many exercise scientists consider RE to be the third critical determinant of distance running performance. In running, carbohydrates and fat are the primary fuels, and the breakdown of each requires oxygen (O2). The energy needed to run at a given pace corresponds to the volume (V) of O2 consumed per unit of time (or VO2). It’s measured by comparing the oxygen levels of inhaled and exhaled air, usually during a treadmill test. Because VO2 depends on body size – larger runners need more oxygen – it’s generally normalised to body mass in kilograms and so is reported as millilitres of O2 per kilogram of body mass per minute: ml/kg/min. The higher this number is, the better. In simple terms, VO2 max is your upper limit of oxygen consumption (your aerobic capacity), lactate threshold is the level of your aerobic capacity that you can sustain for a long time, and running economy (RE) is your efficiency at converting that oxygen consumption into forward motion. For any given pace, the less energy and oxygen you use, the better. So what factors can affect an individual’s RE, and how can you improve yours?

1. Muscle fibre composition

A person’s composition of ‘slow-twitch’ (type I) and ‘fast-twitch’ (type II) muscle fibres is believed to be genetically determined because they change little over time, even when training is drastically altered. But researchers at the University of Texas have shown that cyclists with more slow-twitch fibres tend to use less oxygen to maintain a given power output. While the data for running is less clear-cut because of the more complex biomechanics, it’s reasonable to speculate that RE also reflects one’s muscle-fibre makeup.

2. Joint flexibility

Among reasonably fit runners and walkers showing typical variations inflexibility, it appears that the less flexible ones are more economical on average. This somewhat surprising trend has been explained as a possible consequence of two factors: stiff joints need less muscle force (and thus less energy) to stabilise them; and stiff muscle-tendon units provide superior elastic storage and energy return from footstrike to push-off. It’s possible that, for distance runners, medium flexibility is preferable to very low flexibility (which increases the incidence of injury due to limited range of motion) or very high flexibility (which seems to worsen RE).

3. Body shape

Even though your VO2 measurement is related to body mass, smaller runners tend to use proportionally less oxygen. Also, according to the physics of rotating limbs, mass towards the bottom of the leg requires more energy to move than mass close to the trunk. Elite East African runners tend to be unusually economical, and it’s been suggested that their small overall size and skinny lower legs are partly responsible for this. A 2006 report by Spanish researchers at the University of Madrid indicated that elite runners from Eritrea had REs about 12 per cent better than a comparable group of Spanish runners.

4. Resistance on the run

Just as your car’s fuel economy depends on the driving conditions, your RE is variable according to running conditions. An uphill slope or difficult footing (such as sand, mud or long grass) raises the energy demands of maintaining a given pace. The same is also true of fighting air resistance (for example, running into a stiff headwind requires more energy). In calm air, British studies from the early 1970s show that air resistance has a negligible effect on RE at slow paces. However, a runner churning out 5:20 miles will expend about two per cent of their energy countering air resistance.

Want to boost your running economy? Try including these workouts in your routine.